A standard digital image sensor is a homogenous array of light sensitive cells called pixels. The pixels can include either charge coupled devices (CCD) or complimentary metal oxide semiconductor (CMOS) devices. The individual pixels are monochrome and only detect light, not color. To provide color images, the monochrome pixels are covered with color filters. Typically filters that pass red, green, and blue light are arranged on a cluster of pixels. Color for each pixel cluster is then determined using a coding scheme such as Bayer encoding. Digital camera image sensors do not have a mechanical shutter, but use an “electronic shutter.” Photons incident to the pixels create charge on the pixels. The brighter the light that is incident to a pixel, the more charge that accumulates on the pixel. This charge results in a voltage on the pixel that can be sampled and quantized. The brightness of the resulting image is related to the voltage sampled on the pixel. An electronic shutter clears the charge from all the pixels in the array. The exposure time of the image sensor is the time from the clearing of the array to the sampling of it.
Image sensors are designed to use the linear range of pixel devices such that the voltage response of the devices is proportional to the amount of light incident to the devices for the dynamic range of the pixel devices. The human eye has a very wide dynamic range. In a high contrast scene, the human eye can see detail in the dark and the light areas of the scene. The dynamic range of a digital image sensor is more limited. For a high contrast scene, the image captured by the sensor may have the brighter details overexposed or washed out because incident light caused the pixels to exceed the upper end of their dynamic range, or the image may have the darker details underexposed because the light incident to the pixel did not reach the lower end of their dynamic range.
For a high contrast scene, the camera user is forced to choose to capture either the detail in dark areas of the scene and leave the light areas over-exposed, or to choose to capture the detail in the light areas of the scene and leave the dark areas under-exposed.